1. Field of the Invention
The present invention relates generally to a device for preventing the burning of a coil for a brushless motor, and more particularly, to a device for preventing the burning of a coil and which is capable of controlling the speed of the motor. Even more particularly, the present invention relates to a device for preventing the burning of a coil and controlling the speed of the motor using a common capacitor.
2. Description of the Related Art
In recent years, applications of small DC motors have been expanding at a very rapid pace not only in the audio industry, but also in the information industry because of their excellent controllability. Among such motors, because of the elimination of mechanically contacting parts such as a brush and a commutator and the advantage of longer life, a brushless motor has found increasing applications as an industrial motor which requires particularly high reliability.
Because of this, DC power has been replacing AC power as the driving source for small axial fans over the past several years, and DC axial fans using a brushless motor have come to be used in increasing numbers.
Also, as industrial devices such as computer-controlled devices have come to be constructed in a higher packaging density, there has been an increased need for DC axial fans for forced-air ventilation use. In addition, there has been an increase in the reliability requirements of such fans themselves. That is, in case a motor were to stop for some reason, it is required not only that the motor coil be protected against thermal damage, but also hat a possible accident be prevented by issuing some kind of warning to the control unit of the device. Moreover, the device should be automatically reset to restart the motor for proper rotation after the cause of such stopping has been eliminated.
At the same time, to satisfy the growing demand for quietness in the office, the requirement has arisen for a variable speed motor in which, when the device generates a smaller amount of heat, the rotational speed of the motor is reduced to decrease the airflow from the fan. Alternatively, when the device generates a larger amount of heat, the rotational speed of the motor is increased to increase the airflow.
A device for preventing the burning of a coil for a DC axial fan motor has been developed by the present inventors in U.S. Pat. No. 4,947,091 issued on Aug. 7, 1990. Such prevention is achieved by using the charging/discharging characteristics of a capacitor.
There have been proposals for a variable speed motor which have been presented. However, such proposals are based on the idea of constructing another circuit for the speed control which is separate from existing circuits for the motor. This will cause an increase in the number of external components, and, when such a circuit is mounted on an IC chip, the number of the terminal pins increases, resulting in a significant cost increase.